Quadrupole nuclei can be powerful probes to detect magnetic and lattice fluctuations in solids.
Naturally abundant quadrupole nuclei \(^{63}\) Cu and \(^{65}\) Cu have nuclear spin \(I\)  = 3/2 with different nuclear gyromagnetic ratios \(^{63, 65}\gamma _n\)  (\(^{63}\gamma _n < {^{65}\gamma _n}\) ) and quadrupole moments \(^{63, 65}Q\)  (\(^{63}Q > {^{65}Q}\) ) [1]}.
In general, the longitudinal nuclear spin relaxation theory tells a magnetic relaxation rate \(^{\eta }W_M\propto {^{\eta }\gamma _n}^2\)  and a quadrupole relaxation rate \(^{\eta }W_Q\propto {^{\eta }Q^2}\)  (\(\eta \)  = 63, 65 stands for \(^{63}\) Cu, \(^{65}\) Cu) [2]}.
Thus, it is an experimental criterion to identify the predominant relaxation process whether the isotopic ratio of the \(^{\eta }\) Cu nuclear spin-lattice relaxation rates \(^{\eta }W_1\) 's is \(^{63}W_1/^{65}W_1>\) 1 or \(<\) 1.
Furthermore, the accurate isotopic measurements of \(^{63}W_1\)  and \(^{65}W_1\)  may enable us to quantify how much role each relaxation plays in the nuclear spin-lattice relaxation.
Successful separation of mixed magnetic and quadrupole relaxation is found in isotopic nuclear quadrupole resonance (NQR) experiments of the chain-ladder composite compound Sr\(_{14}\) Cu\(_{24}\) O\(_{41}\)  [3]} and the kagome lattice Heisenberg antiferromagnet Zn-barlowite ZnCu\(_3\) (OD)\(_6\) FBr [4]}.
